Memory devices are typically provided as internal storage areas in computers. The term memory identifies data storage that comes in the form of integrated circuit chips. In general, memory devices contain an array of memory cells for storing data, and row and column decoder circuits coupled to the array of memory cells for accessing the array of memory cells in response to an external address.
One type of memory is a non-volatile memory known as flash memory. A flash memory is a type of EEPROM (electrically-erasable programmable read-only memory) that can be erased and reprogrammed in blocks. Many modern personal computers (PCs) have their BIOS stored on a flash memory chip so that it can easily be updated if necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in wireless electronic devices because it enables the manufacturer to support new communication protocols as they become standardized and to provide the ability to remotely upgrade the device for enhanced features.
A typical flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating-gate field-effect transistor capable of holding a charge. The cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed on an individual basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge on the floating gate.
Memory devices are typically formed on semiconductor substrates using semiconductor fabrication methods. The array of memory cells is disposed on the substrate. Isolation regions formed in the substrate within the array, e.g., shallow trench isolation, provide voltage isolation on the memory array by acting to prevent extraneous current flow through the substrate between the memory cells. High-voltage circuitry, such as for accessing, programming, and erasing the memory cells, e.g., select circuitry having high voltage pumps, etc., is also disposed on the substrate at a periphery of the memory array. Isolation regions formed in the substrate at the periphery provide high-voltage isolation at the periphery by acting to prevent extraneous current from flowing through the substrate between the high-voltage circuitry and the memory array.
The isolation regions are often formed within the array and the periphery concurrently. One problem with this is that the periphery and array trenches that contain the isolation material, e.g., dielectric material, tend to have substantially the same depth that is limited by the depth of the array trenches because the aspect ratio (trench depth to trench width w) of the array trenches is larger than that of the periphery trenches because of the relatively small spacing between memory cells, especially for arrays having high-densities of memory cells. Trenches with higher aspect ratios become more difficult to fill. However, this trench depth for the periphery trenches is often insufficient for preventing extraneous current flow through the substrate between the high-voltage circuitry at the periphery and the memory array. Alternatively, the periphery trenches, with their larger widths, are often made deeper than the array trenches. However, this often results in extra and often more complex fabrication steps.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative methods for forming isolation regions of memory devices.